As of Eclipse project build I20080305 (shortly before Eclipse 3.4/Ganymede M6), the Eclipse SDK contains a new provisioning system called Equinox p2. P2 replaces Update Manager as a mechanism for managing your Eclipse install, searching for updates, and installing new functionality. This document will help you getting started with p2. If you want to explore some of the capabilities of p2 that are not exposed to end users in the Eclipse SDK, see also Equinox p2 Getting Started Admin UI.
p2 user interface
In the workbench, p2 replaces the Help > Software Updates menu entry from Update Manager. The resulting dialog shows the features that are installed, and the features that are available to install. You can add additional update sites by clicking Manage sites in the Available features pane. Select any set of available features and click Install to install them into the current running system.
Before p2, many Eclipse users circumvented Update Manager and installed new plug-ins by dumping them in the eclipse/plugins/ directory and restarting with the -clean command line argument. There are many drawbacks to this "wild west" approach that we won't get into here, but suffice it to say that p2 does not have this promiscuous behavior of attempting to install every plug-in it can get its hands on. By contrast, p2 employs a much simpler, more predictable, and ultimately much more flexible startup routine.
When you install a p2-enabled Eclipse application, you will notice some new files and directories that didn't exist before. Here is a subset of the Eclipse install tree with some of this new content highlighted:
eclipse/ configuration/ config.ini org.eclipse.equinox.simpleconfigurator/ bundles.info dropins/ features/ p2/ plugins/ eclipse.exe eclipse.ini ...
The file bundles.info contains a list of all the plug-ins installed in the current system. On startup, all the plug-ins listed in this file will be included. Any extra plug-ins in the plugins directory or elsewhere are ignored. If you really want to force Eclipse to startup with a particular set of bundles installed, you could manually edit this file to have the contents you need. However, unless you're just hacking around or testing, editing this file is not recommended. However, it's useful to know about this file so you can see exactly what is installed in the system you are running.
The new dropins folder is where you can drop in extra plug-ins if you don't want to use the p2 user interface. A subtle twist on old behavior here is that plug-ins and features added to the dropins folder are properly installed into the system rather than being forced in. This means p2 has an opportunity to confirm that the new plug-in doesn't conflict with other installed plug-ins, and it can even go out and fetch any missing prerequisites of the newly dropped in plug-ins. This also means you can later use the GUI to install extra functionality that depends on the plug-ins in the dropins folder, since p2 knows about them and can reason about their dependencies. In other words, new plug-ins installed via the dropins folder behave exactly like plug-ins installed via the user interface.
Interaction with legacy Update Manager
In Eclipse platform version 3.4, the old Update Manager still exists under the covers for backwards compatibility. You can even re-enable the old Update user interface by enabling the "Classic Update" capability on the General > Capabilities preference page.
However, users will rarely have a need for enabling Update Manager, because p2 is able to install from any update site that was designed for Update Manager. Just go into Help > Software Updates > Available Features > Manage sites, and add the update site you want to use. The features from that site will be added to the available features list, and from there you can install them into the system.